A man who murdered a mother of three during a violent home invasion south of Adelaide has been jailed for at least 15 years.

Karen Hodgson, 46, was asleep in her Sturt home in April 2010 when armed men bashed in her front door.

One of the men, 27-year-old Kelly Lee Pearman, was forcing Ms Hodgson to walk to a bedroom at gun point when the weapon accidentally fired and shot her in the back.

The Supreme Court was told the men intended to steal cannabis plants growing in the home's garage.

In sentencing, Justice Margaret Nyland said she did not understand why the gun was loaded when it was only intended to frighten but not kill anyone.

She accepted Pearman's argument that the gun went off accidentally.

"It is accepted you didn't intend to kill Ms Hodgson or anyone else on that night and her death was the result of the accidental discharge of the firearm," she said.

"To break into someone's house in the middle of the night, armed with a loaded gun, is a very serious offence, even if nobody was killed.

"I have some difficulty understanding why the gun was loaded when it was only intended to frighten."

Justice Nyland said good reason existed to set a non-parole period lower than the mandatory 20-year sentence because Pearman had shown remorse and accepted responsibility for the crime by pleading guilty.

She jailed Pearman for life with a 15-year non-parole period.

Another man who admitted to the murder, Scott Mundy, 32, is expected to be sentenced tomorrow.